of HET Data Acquisition
What targets can HET access?
The HET can observe 81% of the sky that reaches elevation above airmass=2.5 from the vantage of 30.68 degrees north latitude. That declination range is about -11 to +72. The target airmass at observation time can range between 1.14 and 1.33, with the typical value being 1.22.
When can the HET service observer access them?
Unlike conventional telescope observing, which is naturally organized by the progression of target right ascensions, HET observing is naturally organized as sequencing of target "tracks." A track consists of all the details in connection with the celestial transit (or at extreme declination: culmination) across the HET "donut" (almucantar) of pointing accessibility on the sky.
This is not so restrictive, for two reasons. A database program organizes the track information (particularly track start, middle, end) for convenient oversight. Also, there is in the north and south azimuth quadrants, where celestial motions are more horizontal than vertical, considerable leeway for setting the structure azimuth displaced from the transit azimuth in order to obtain nearly (>90%) the cumulative pupil available in the transit geometry. This extends accessibility to earlier and later sidereal times.
When will your program get access over competing programs?
After sheer celestial accessibility, the dominant factor is TAC priority (0 strongest to 4 weakest), which most influences the execution order among the many science targets of a given trimester. The consequence is that strong priority (0 or 1) targets generally receive visits early in the trimester or as soon as accessible, middle priority (2 or 3) targets being more concentrated toward mid-trimester, and low priority targets significantly concentrated toward the trimester end.
The HET design imposes certain constraints on the timing of accessing parts of the celestial sky, compared with a conventional telescope of similar aperture. Full productivity is however redeemed by means of compensating design features: the queue observing method and a large degree of flexibility in optimizing efficiency via the on-the-fly choice of the most efficiently observed targets, by folding in a number of time-variable factors in pursuit of optimizing the use of the telescope at all times.
Thus unlike a conventional telescope of similar aperture, which can opt to observe in a purely monotonic priority sequence, sidestepping hard choices by resorting to higher and higher airmasses ("target choice 1 AND target choice 2"), the HET requires a more continual and active discrimination between currently available targets ("target choice 1 OR target choice 2"). This is no real handicap so long as the time-dependent choice modifying factors are given due consideration. The trade-off is that priority, while dominant, is not the sole determinant factor at any instance, but not to the extent that it becomes meaningless.
With minimal elaboration, the time-dependent target choice (non-priority) factors include the following:
Disfavorable discrimination factors:
- Very high wind, permitting visits to certain azimuth targets instead of others.
- Degree of sky transparency, relative time smoothness of the sky transparency, sky brightness, and seeing, permitting certain science and not other.
- Unsteady conditions, in general tending to permit shorter duration targets, instead of long duration ones that risk wasteful failures.
- Detector equipment status, permitting certain science and not other.
- Leads and lags in the time sequence of track accessibility, permitting seamless (no wait) starting on some targets, and thus preempting the starting of others.
- Leads and lags relative to the time of optimal pupil, such that the visit to some targets would now be highly efficient, whereas other targets will enjoy comparable efficiency only on a future track or night (and there is ample expectation of the latter).
- An already excessive number of instrument configurations with time consuming calibrations (that have to extend well into the daytime hours in conflict with other uses of the facility), disfavoring engaging an additional instrument configuration that particular night.
Favorable discrimination factors:
- Scientifically based time criticality of the observation, e.g. synoptic targets that are becoming overdue relative to the visit cadence specified, or pulsation or orbital phasing requiring that particular track time.
- Diminishing availability (calendrical, lunar cyclic) time criticality of the observation.
- Taking advantage of time dependent instrument configurations, e.g. a grism due to be switched out or an MRS setup that is currently set in position.
- Targets with many visits requested, as they near completion can be favored to avoid the risk of a major incompletion case at trimester end.
- If the queue is very congested, targets that permit splitting of sub-exposures between different tracks can sometimes permit execution.
- All else being equal, sequencing targets lying within a small (3deg)
azimuth range, diminishes setting overheads.
- Targets requiring the absolutely rarest, best sky conditions, may be purposely matched to those chance occurrences.
What are the HET comparative advantages?
- The telescope geometry enforces a scheduling discipline which nets an advantageously low typical airmass at time of observation.
- It is easy to fine tune observing to a desired cadence or pulsation or orbital phasing.
- Poor or borderline conditions do not cause any resource wastage, unlike the expenses incurred for comparable visitor mode observing.
- Observing efficiency and reliability benefit from continual practice and upgradings.
How different are typical HET observing procedures?
HET observing broadly resembles conventional telescope observing (with an observer/operator pair) except that the target choice optimization has to be planned more proactively. The constraints on HET productivity are slightly different for two reasons. Firstly, the increased pupil cross-section in the middle reaches of a track compared with the extremes, drives a more detailed analysis of the scheduling. Secondly, there being no "Plan B" of picking up targets from just anywhere on the sky, there is more inherent risk of incurring queue gaps (in time and azimuth). Although there is no official "filler target" mechanism, as the queue wanes arrangements must sometimes be considered to prevent any time or wind-azimuth dependent "hole" being forced upon the telescope usage. Finally, being pure queue service mode, there can be significant PI daily (and with TOOs, nightly) input to the process via communication channels.
Last updated: Sun, 08 Jan 2012 09:01:19 +0000 caldwell